The present invention relates to offset printing and particularly to the configuration of the rubber cover of the offset cylinder in relation to the printing plates.
In a lithographic offset printing press, the lithographic printing plates are attached to a plate cylinder which rotates in contact with the rubber covered blanket cylinder. The ink from the lithographic printing plates is transferred to the rubber blanket and from there to the paper. The manufacture of lithographic printing plate stock normally involves the processing (graining, anodizing, coating, etc.) of aluminum webs which are the width of several printing plates. After processing, these webs are cut transversely with a shear to the desired plate length and slit lengthwise to the desired plate width to form the individual printing plates. The shearing and slitting can be done in either order as desired. For one example, a typical web is the width of or slightly greater than the width of three Metro size newspaper plates which are in the range of 135/8 to 137/8 inches wide. The stock of aluminum web is normally in the range of 41 to 45 inches wide. Two slits are made in the web to form the three plates and the outer edges of the web are also usually slit depending on the exact width of the desired plates in relation to the width of the aluminum web stock and to eliminate imperfect edge coatings.
The process of slitting the web longitudinally is done with rotary slitters. The slitting process of the anodized and treated aluminum substrate exposes the bare metal at the edges of the plates. This bare metal is oleophilic and may be susceptible to ink pick-up from the inking roller and the transfer of that ink to the offset blanket and then to the paper. This can result in an undesirable ink stripe down the edges of the printed page. The slitting process also may create burrs on the slit edges with the burr on one side of a slit extending up on the face of the plate (up-burr) and the burr on the other side of that slit extending down on the back of the plate (down-burr). The height of the burrs varies and is primarily determined by the sharpness of the rotary slitters. The height can usually be kept less than 0.001 inches with a reasonable sharpening schedule. Although the down-burrs on the back of the plates are not a particular problem, the up-burrs present two problems in the offset printing process. The first problem that is created by the burrs on the printing plates is that the up-burrs exacerbate the ink pick-up problem at the plate edges. The second problem is that these up-burrs can damage the offset blankets. To totally eliminate the up-burrs on the front face of the plates would require additional slitting steps to produce all down-burrs. However, this produces scrap plate material which, although a small percentage of the total plate material, would be excessively costly for the mass production of lithographic printing plates. Also, elimination of the up-burrs does not eliminate the ink pick-up problem at the edges caused by the bare metal.
Typical newspaper printing presses have plate cylinders which are either two or four plates wide. The plates are mounted on the cylinders with a space of about 1/8 inch between plates. The offset blanket which picks up the ink from the multiple plates could be made slightly narrower than the outside edges of the plates on the ends of the cylinder such that the blanket will not engage these oleophilic outside edges or the burrs that may be present. However, the oleophilic edges and the up-burrs that may be on the edges between adjacent plates on the cylinder will still engage the blanket and may transfer ink and damage the blanket.